1. Field of Invention
The present disclosure relates to sampling methods and apparatus, and more particularly to sequencing and averaging multiple sampling methods and systems.
2. Prior Art
Continuous online analysis of components in liquid and gas streams is a common practice in industry today. For example, routine sampling and analysis is conducted in power plant smokestacks, liquid waste streams, industrial process streams, the head space in storage vessels and many other sources. Because analytical monitors and instrumentation are quite expensive, it is often desirable to share an analyzer bank with multiple sample streams. This is usually accomplished by one of two methods—sequencing or averaging.
In sequencing, the analyzer tests each sample in sequence, on a time share basis. A controller or multiplexer system may utilize a programmable logic controller (PLC), distributed control system (DCS) or a computer to sequence through multiple sample chambers, each obtaining samples via valve-controlled orifices. An example of a sequencing sampling system in shown in U.S. Pat. No. 4,325,910 (Jordan). Averaging systems involve feeding multiple input samples into a central manifold or header and mixing the samples prior to analyzing the mixture. A control device, such as a needle valve, is used to precisely control the flow of each sample into the manifold. The combined mixture is then analyzed to provide an average reading for all of the samples.
Either of the above two methods works fairly well in the absence of an upset condition. However, when a problem arises with one of the sampled streams, both prior art systems have difficulty in quickly locating the stream that is out of specification. In the case of sequencing, the upset condition will not be detected during the time that the analyzer is testing other samples. If the streams being analyzed are potent, toxic, flammable, radioactive or otherwise dangerous, the delay in detecting an upset condition could have serious consequences. An averaging system could also involve a substantial delay in finding an upset condition, because the samples are all mixed before being analyzed. Moreover, it may be difficult to even detect the presence of a problem sample because each sample is substantially diluted with other samples before being analyzed. Accordingly, new methods and apparatus are needed to quickly and accurately locate an upset condition in a stream flow using a multiple stream analyzer.